Infrared detection systems which operate over the 2 to 20 .mu.m wavelength region, and which are required to have an optical resolution of 1 mrad or less, conventionally employ either a germanium refracting objective or a catadioptric objective such as a Schmidt system. The optical system is used in conjunction with a single infrared detector or array of detectors, associated signal processing electronics, and a suitable target indicator device or display.
In order to detect targets over a substantial field of view, the small instantaneous field of view defined by the focal length of the optical system and the size of individual detectors may be optically scanned through the total search field of view. This may be achieved, for example, by means of rotating optical components which generate a line-by-line rectangular raster scan.
Alternatively, a large number of detectors may be employed without scanning, i.e. a staring focal plan array system. This technique suffers from various limitations arising from the finite spacing between adjacent detectors.
A disadvantage of refractive and catadioptric system is their high unit cost. Furthermore, optomechanical scanning techniques which employ motor-driven components moving at high speed to generate the desired search field, add to system complexity and overall cost.